Field of the Invention
The invention relates to a high-temperature fuel cell and to a stack of high-temperature fuel cells.
It is known that in the electrolysis of water, the water molecules are decomposed by electric current into hydrogen (H.sub.2) and oxygen (O.sub.2). In a fuel cell, this process proceeds in reverse order. An electrochemical combination of hydrogen (H.sub.2) and oxygen (O.sub.2) into water produces electric current with high efficiency and, if pure hydrogen (H.sub.2) is used as the gaseous fuel, without emitting pollutants and carbon dioxide (CO.sub.2). Even with an industrial fuel gas, such as natural gas or coal gas, and air (which may additionally be enriched with oxygen (O.sub.2)) instead of pure oxygen (O.sub.2), a fuel cell generates markedly less pollution and less carbon dioxide (CO.sub.2) than other energy producers that use fossil fuels. The industrial application of the principal of the fuel cell has led to various embodiments using different kinds of electrolytes and with operating temperatures between 80.degree. C. and 1000.degree. C.
Depending on their operating temperature, the fuel cells can classified as low-temperature, medium-temperature, and high-temperature fuel cells, which in turn are distinguished by different technical embodiments.
In the high-temperature fuel cell stack composed of many high-temperature fuel cells (in the professional literature, a fuel cell stack is also called a "stack"), below an upper composite printing circuit board that covers the high-temperature fuel cell stack, there are in order at least one guard layer, a contact layer, an electrolyte electrode unit, a further contact layer, a further composite printing circuit board, and so forth.
The electrolyte electrode unit includes two electrodes and a solid electrolyte, embodied as a membrane, disposed between the two electrodes. Each electrolyte electrode unit located between adjacent composite printing circuit boards, together with the contact layers immediately adjacent the electrolyte electrode unit, form one high-temperature fuel cell, which also includes the sides of each of the two composite printing circuit boards adjoining the contact layers. This type of fuel cell and other types are known for instance from the "Fuel Cell Handbook" by A. J. Appleby and F. R. Foulkes, 1989, pp. 440-454.
The components (such as the two metal composite printing circuit boards) of the high-temperature fuel cell are joined together for operation in subregions (the so-called joining regions; in the case of the composite printing circuit boards, for instance in the edge region). The term "joining" is understood to mean placement together in a fitting way or combining of work pieces or materials by various methods (such as screwing, riveting, welding, and so forth). Correspondingly, the parting (opening) between two work pieces (in this case components) to be joined is called a seam.
Various demands are made of the layer in the joining region that closes the seam between the components. The layer must have adequate gas-tightness. With a joining region disposed for instance in the peripheral region of the fuel cell (also called an outer joining region), it is thus assured that the fuel media, which in the fuel cell are in the gaseous state, cannot escape from the fuel cell to the environment. If the joining region is disposed in the interior of the fuel cell (internal joining region), then mixing of various fuel media (such as hydrogen (H.sub.2) and oxygen (O.sub.2)), is for instance prevented.
The layer must furthermore be invulnerable to an elevated pressure. The fuel media in the fuel cell will have an elevated pressure relative to the ambient atmosphere.
The layer must furthermore have adequate mechanical stability in the face of mechanical stresses that occur in the fuel cell. Temperature changes, for instance when a high-temperature fuel cell stack, which as a rule is composed of at least 40 fuel cells, is turned on and off, or changes in operating temperature (which depending on requirements can be between 600 and 1000.degree. C.) exert considerable mechanical forces on the joining region. When the high-temperature fuel cell stack is assembled as well (or more precisely when the stack mechanically subsides), mechanical stresses cannot be avoided.
The material forming the layer in the outer joining regions should furthermore have added electrical insulation. An electrical short circuit between the components that are joined together by the layer must be avoided, because that would reduce the efficiency of the fuel cell. Electrochemical stability must exist, so that the slight electric current loss that nevertheless flows out via the layer will not destroy the material forming the layer and cause the layer to leak and allow gas to flow through.
The material forming the layer must furthermore be stable in the face of a chemical reaction with the fuel media inside the fuel cell. The fuel media in the fuel cell are in the form of reducing humid gasses, which can act chemically aggressively on the material of the layer. This can again cause leaks in the joining region of the fuel cell. Chemical compatibility with the material forming the components to be joined must be assured as well.
From International Patent Application WO 96/17394, layers for joining components of a high-temperature fuel cell are known that contain plies of a glass and plies of a ceramic. The plies of glass or ceramic are available in the form of prefabricated frames. Manufacturing the frames involves high cost, because the frames must be manufactured to high precision (in the .mu.m range). In addition, assembling the various plies, because of the large number of fuel cells in a stack, requires considerable effort for adjustment and measurement. To obtain sufficiently stable glass frames, the glass is provided with additives (such as arsenic oxide (As.sub.2 O.sub.3)). The additives can impair the quality of the joining region with regard to the properties required.
The ceramic frames are preferably produced by atmospheric plasma spraying or by vacuum plasma spraying. The frames produced in this way do not have adequate gas-tightness when used for relatively long times in the fuel cell.